STABLE ISOTOPIC STUDY OF PRECIPITATION AND SPRING DISCHARGE IN SOUTHERN NEVADA

Precipitation was collected in southern Nevada on a semi-regular monthly basis at 14 locations for 6 years for stable isotopic analysis. The total elevational range of the collection sites was 1270 m and the total geographic range was 65 km. The average yearly difference in delta-O-18 of precipitation from all sites was smallest and amounted to only 2.3 parts-per-thousand, whereas the average difference based on location (elevation) was only 3.7 parts-per-thousand. The largest difference in delta-O-18 was between enriched summer and depleted winter precipitation and amounted to more than 13 parts-per-thousand. The precipitation record shows two time-based regimes. For the first 3 years of collection, the precipitation was highly variable with several large events and several dry periods. During the last 3 years of collection, the precipitation was much more even with no large events. However, there is no correlation between the variability of the amount of precipitation and the stable isotopic composition of precipitation. In addition, the delta-O-18 composition and discharge of two springs, Whiterock Spring and Cane Spring, issuing from perched water tables, were monitored for 5 years on a similar basis as for the precipitation. During the first 2.5 years of collection, both volume of discharge and delta-O-18 responded to large precipitation events. However, during the last 2.5 years of collection when precipitation moderated in amounts, but not in stable isotopic composition, the springs displayed not only less variation in discharge, but in stable isotopic composition as well. Summer precipitation may be involved in groundwater recharge, as during the first 3 years of collection, some of the events to which the springs responded were summer events. Moreover, the average delta-O-18 of winter precipitation is less than that of the spring discharge, even when the water from large summer events is absent from the spring discharge.